This invention relates to implantable cardiac pacemakers, and particularly to rate-responsive cardiac pacemakers. More particularly, this invention relates to a system and method for automatically determining the slope of a transfer function that is used by a rate-responsive pacemaker to determine an appropriate heart rate in accordance with metabolic demands.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate--typically a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced pacemakers, the rate could be set remotely after implantation. Such pacemakers were known as "asynchronous" pacemakers because they did not synchronize pacing pulses with natural cardiac activity.
Early advances in pacemaker technology included the ability to sense the patient's natural cardiac rhythm (i.e., the patient's intracardiac electrogram, or "IEGM"). This led to the development of "demand pacemakers"--so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, the demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable because pacing pulse inhibition extends battery life.
Demand pacemakers allow physicians to telemetrically adjust the length of the escape interval, which has the effect of altering the heart rate maintained by the device. However, in early devices, this flexibility only allowed for adjustments to a fixed programmed rate, and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, a patient receiving therapy from an early demand pacemaker was paced at a constant heart rate--regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, the patient was subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because the heart rate was forced by the pacemaker to remain constant.
The adverse effects of constant rate pacing lead to the development of "rate-responsive pacemakers" which can automatically adjust the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increases the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include processing circuitry that correlates measured physical activity to an appropriate heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the transition rates between the minimum heart rate and the maximum heart rate are parameters that may be telemetrically adjusted to meet the needs of a particular patient.
One approach that has been considered for enabling rate-responsive pacemakers to determine an appropriate heart rate involves the use of a physiological parameter that reflects the patient's level of metabolic need. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks (such as slow response time, unpredictable emotionally-induced variations, and wide variability across individuals) render the use of these physiological parameters difficult, and accordingly, they have not been widely used in practice.
Rather, most rate-responsive pacemakers employ sensors that transduce mechanical forces associated with physical activity, the level of physical activity being indicative of the patient's level of metabolic need. These activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential when a mechanical stress resulting from physical activity is experienced by the sensor. By analyzing the signal from a piezoelectric activity sensor, a rate-responsive pacemaker can determine how frequently pacing pulses should be applied to the patient's heart.
Piezoelectric elements for activity sensors are commonly formed from piezoelectric crystals, such as quartz or barium titanite. Recently, however, activity sensors have been designed which use thin films of a piezoelectric polymer, such as polyvinylidene fluoride (commonly known by the trademark KYNAR, owned by ATOCHEM North America) as the transducing element, rather than the more commonly used piezoelectric crystals. Activity sensors so designed are described in copending, commonly-assigned U.S. patent applications Ser. No. 08/059,698, filed May 10, 1993, entitled "A Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based Sensor and Method of Fabrication," and Ser. No. 08/091,850, filed Jul. 14, 1993, entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," which are hereby incorporated by reference in their entireties.
A variety of signal processing techniques have been used to process the raw sensor signals provided by activity sensors. For example, in one approach, the raw signals are rectified and filtered. Alternatively, the frequency at which the highest peaks in the signals occur can be monitored. Regardless of the particular method used, the result is typically a digital signal that is indicative of the level of sensed activity at a given time. In one preferred approach, the digital signal is produced by repeatedly integrating the raw sensor signals until a predetermined threshold value is reached. Each time the threshold is reached, a digital trigger pulse is generated. A counter is used to count the number of trigger pulses that occur in a fixed period of time (e.g., the number of trigger pulses that occur during an approximately 100 ms period within each heartbeat interval). The count reached at the end of the fixed period of time is provided to processing circuitry in the pacemaker, which processing circuitry typically includes a microprocessor.
The microprocessor then uses the count signal to produce a sensor level index signal that represents the patient's activity level. The appropriate rate at which the patient's heart is to be stimulated (known as the sensor-indicated rate) is determined by applying a transfer function to the sensor level index signal. The transfer function defines a sensor-indicated rate for each possible sensor level index signal.
An example of a rate-responsive pacemaker in which a transfer function is used to calculate the sensor-indicated rate is described in commonly-assigned U.S. Pat. No. 5,074,302 of Poore et al. ("the '302 patent"), which is hereby incorporated by reference in its entirety. As described therein, when relatively little activity is detected, the sensor level index signal is ordinarily below a low activity threshold. When the sensor level index signal is below the low activity threshold, the sensor-indicated rate is set to a base pacing rate (e.g., 60 beats per minute (bpm)), as defined by the transfer function. At high levels of measured activity, the sensor level index signal may exceed a high activity threshold. When this occurs, the sensor-indicated rate is limited to a maximum pacing rate, so that the patient's heart is not stimulated too rapidly. If the value of the sensor level index signal falls between the low and high activity thresholds, the pacemaker applies pacing pulses to the patient's heart in accordance with the rate determined by the transfer function, generally at a rate somewhere between the base pacing rate and the maximum pacing rate.
Typically, for sensor level index signals between the low and high thresholds, the transfer function is linear. The slope of the transfer function determines increases (or decreases) in the pacing rate corresponding to a given increase (or decrease) in the sensor level index signal. The larger the slope, the more dramatically the pacing rate will increase (or decrease) for a given sensor level.
The slope of the transfer function in typical rate-responsive pacemakers is telemetrically adjustable by a physician, so that the operation of a pacemaker can be tailored to suit an individual patient's needs. However, the task of manually selecting an appropriate slope for the transfer function is labor intensive and time consuming. Generally, the process of manually selecting the slope of the transfer function requires the patient to walk about for a period of time after each adjustment to the slope, so that the physician can monitor the performance of the pacemaker under each new slope setting.
In order to avoid the drawbacks associated with manual slope selection, pacemakers have been designed which can automatically adjust the slope of the transfer function, as described in the '302 patent. In order to automatically adjust the slope, the pacemaker described in the '302 patent measures the level of patient activity for a predetermined period of time. Because most patients are generally at rest for most of the day, the average of the sensor level index signals is approximately the same as the sensor level index signal for the patient at rest. Therefore, as described in the '302 patent, by calculating the average sensor level index signal over the predetermined period of time, a sensor level index signal can be generated that is appropriate to use as the low activity threshold. Similarly, a satisfactory determination of an appropriate value for the high activity threshold can be made by averaging some of the highest sensor level index signals that are measured during the same predetermined period of time. In this way, the pacemaker of the '302 patent is capable of automatically updating the low and high activity thresholds, which in turn has the effect of defining the slope of the transfer function.
Although the approach for adjusting the slope of the transfer function described in the '302 patent is generally satisfactory, it would be desirable if there were an approach for calculating and updating the slope of the transfer function that would allow a pacemaker to take into account certain aspects of a patient's condition that have previously not been addressed. For example, it would be desirable if the slope of the transfer function could be automatically determined based on the physician's selection of the base pacing rate and the maximum pacing rate, along with the patient's activity profile. It would also be desirable if the pacemaker could determine when the patient has been unusually inactive for an extended period of time, so that the pacemaker does not set an inappropriately steep slope for the transfer function. It would further be desirable if the pacemaker could set the slope of the transfer function in accordance with the patient's regular exercise routine.